1. Field of Invention
This invention relates to a solids mixing, storage and conveying, and, in particular to an improved static mixer, hopper and conveyor to introduce a continuous flow of granular material such as polycrystalline silicon into a high temperature furnace for the production of single crystal ingots.
2. Brief Statement of the Prior Art
Single crystal silicon is the basic substrate used for virtually all semiconductor devices. Other single crystal materials which are also finding applications in semiconductor devices are Periodic Table Group III elements, particularly in combination with Group VI and Group V elements, e.g., germanium and gallium arsenide. These materials are synthetically produced in a purified and perfect, single crystal state. The method traditionally used for such production has been the Czochralski method. In the Czochralski method, polycrystalline material, such as silicon, is melted and maintained in a molten state in a quartz crucible. The quartz crucible is mounted in a heated furnace, usually supported by a graphite cup. A seed crystal of silicon is dipped in the molten silicon and is slowly withdrawn, forming a cylindrical boule of single crystal silicon. The boule and crucible are rotated counter-rotationally to promote uniformity of silicon properties and distribution of impurities and dopant additives within the silicon.
The Czochralski process is conducted batch-wise and inherent limitations of batch-wise processing cause, or promote, variations in properties and composition of the silicon boule. Dopants such as phosphorus or boron are usually added to the silicon melt to impart desirable semiconductor properties to the silicon wafers which are sliced from the cylindrical silicon boule. The dopants tend to concentrate in the molten pool as the pool is deplenished by the forming of the boule. Additionally, oxygen is introduced into the molten pool from reaction of the silicon melt with the surfaces of the quartz crucible. As the level of the molten pool decreases during the batch processing the wetted surface area decreases, resulting in a continuous decrease in oxygen concentration in the melt.
These and related inherent limitations in the Czochralski process have led to the improved processes which are described in U.S. Pat. No. 4,659,421 and 5,314,667. In these improved processes, a shallow molten pool of silicon is maintained in a crucible to reduce the variation of dopant and oxygen contents in the cylindrical boule withdrawn from the pool. Polycrystalline silicon is continuously introduced into a silicon replenishment zone which is separated from the crystal growth zone, where the boule is formed. In the U.S. Pat. No. 5,314,667 patent, the replenishment zone is an annular zone surrounding the central crystal-growth zone.
Silicon is continuous introduced into the furnaces of these improved process; in the U.S. Pat. No. 4,659,421 patent, the silicon is supplied as a solid rod to stir the molten silicon pool, while in the U.S. Pat. No. 5,314,667 patent, granular silicon is introduced into the replenishment zone.
It is also difficult to introduce granular material into a high temperature furnace at closely controlled feed rates, as required to avoid upsetting the process conditions within the furnace. A common problem with all silicon crystal growth furnaces is the difficulty of obtaining intimate distribution of the minor quantities of dopant throughout the polycrystalline silicon supplied to the furnace so that variations in dopant concentration are avoided in the single crystal boule.